Various contrivances are conventionally proposed regarding an exhaust gas deNOx apparatus for a diesel engine, and one disclosed in Japanese Patent Application Laid-open No. 7-279718 is cited as an example thereof.
According to Japanese Patent Application Laid-open No. 7-279718, an NOx adsorber is placed in an exhaust passage of an engine, and an exhaust gas recirculating (EGR) circuit having an EGR control valve for controlling an amount of flowing exhaust gas is provided. The NOx adsorber absorbs NOx in a normal condition, and it releases NOx when an air fuel ratio inside a combustion chamber becomes rich, and reduces NOx by CO and HC. When the air fuel ratio inside the combustion chamber is to be made rich, the EGR control valve is opened to allow exhaust gas to recirculate, an amount of intake air is decreased, and injection fuel is increased to thereby make the air fuel ratio rich. At the same time, a fuel injection amount is further increased so that engine output torque does not change.
However, in the above-described constitution, a large amount of NOx is contained in exhaust gas because the EGR control valve is closed during normal operation, and therefore it is necessary to increase the size of the NOx adsorber, which makes a space area larger in the case of a large-sized engine, thus causing the constitutional disadvantage, and also causing the disadvantage of cost being high. In addition, when fuel is increased under a heavy load, there arises the fear that inner pressure of cylinders and exhaust gas temperature are increased and reliability and durability of the engine are impaired.